Electrodynamic transducers used in professional applications require unaltered performance even at very high excitation levels. When excited with a high level of input signal, the electrical current running through the voice coil causes an increase in the voice coil temperature which leads to a higher voice coil resistance and, as a result, higher electric losses. Eventually, very high temperatures will lead to a complete failure of the voice coil assembly. Higher electric losses in turn cause so-called thermal compression, decrease of the sound pressure level, and decrease of electrodynamic damping. This is explained by the fact that most electrodynamic transducers work with amplifiers that are sources of voltage. The electric losses hence decrease the voice coil current and correspondingly the force driving the voice coil.
Many loudspeaker systems require an acoustical separation of the different transducers. The separation of mid-range transducers (air sealing) is usually done by putting them in a separate sealed enclosure, such as a back cover. This prevents undesirable modulation of the diaphragm by sound pressure produced by the low frequency transducers (woofers). The air in such a back cover does not allow much dissipation of the heat generated by the transducer since there is no thermal exchange with the air outside the loudspeaker system. Due to the small volume of the back cover, the voice coil temperature may significantly increase.
The air trapped in the back cover also acts as a nonlinear acoustic compliance, which can have a significant influence on the overall mechanical stiffness of the transducer (A. Voishvillo, “Nonlinear Versus Parametric Effects In Compression Drivers”, 115th AES Convention, preprint 5912, 2003, New York). Due to the nature of the air, the stiffness changes differently for inward or outward movement of the diaphragm or cone. The relation between the stiffness of the air Kma and the displacement of the cone x can be described as:
                                          K            ma                    ⁡                      (            x            )                          =                              γ            ⁢                                                  ⁢                          p              0                        ⁢                          V              0              γ                        ⁢                          S              d              2                                                          (                                                V                  0                                +                                  x                  ⁢                                                                          ⁢                                      S                    d                                                              )                                      γ              +              1                                                          (        1        )            where Sd is the effective surface area of the cone, V0 is the volume of the back cover, p0 is the static air pressure in the back cover and γ is the adiabatic index of air.
Expression (1) can be also written as:
                                          K            ma                    ⁡                      (            x            )                          =                              ρ            ⁢                                                  ⁢                          c              2                        ⁢                          V              0              γ                        ⁢                          S              d              2                                                          (                                                V                  0                                +                                  x                  ⁢                                                                          ⁢                                      S                    d                                                              )                                      γ              +              1                                                          (        2        )            where ρ is the density of air, and c is the speed of sound.
The mechanical stiffness of a transducer in a sealed enclosure includes the stiffness of the transducer's suspension Kms and the stiffness of the sealed back cavity Kma:Km(x)=Kms(x)+Kma(x)  (3)
This way, a small volume of the air cavity not only has a significant influence on the overall stiffness of the system, but also on its nonlinearity.
In order to provide an acoustical insulation, a back cover as illustrated in FIG. 1 is a common prior solution. Its volume can be adjusted to fulfill requirements for the mechanical behavior of the transducer and the space in the loudspeaker system. As mentioned above, the small amount of air in the rear cavity surrounding the motor does not provide good heat transfer from the motor to the outside environment. Thus, the motor will heat up much faster in the back cover than without it.
Another prior option has been to seal the frame of the transducer which is attached to the top plate of the motor. This way, most of the motor will be outside of the sealed enclosure as illustrated in FIG. 2. This design provides better heat dissipation since most of the motor parts have direct contact with the air outside of the cavity. Additionally, heat transfer may be improved by attaching a heat sink on the back of the motor. This design also makes a separate back cover redundant because the acoustic isolation is provided by the sealed frame.
The disadvantage of the design in FIG. 2 is that the volume of the air cavity is limited by the transducer dimension. The small volume between the frame and the cone leads to a significant increase of the stiffness which limits the lower end of the frequency range where the transducer might be used. Additionally, the nonlinearity of the air stiffness may increase the distortion generated by the transducer.